A rock bolt is an anchor bolt used for stabilizing a rock excavation during construction of a tunnel, for example a transportation tunnel or a mining tunnel. During construction of a tunnel, a cavity is created in a mass of rock. The interface between the remaining portion of the mass of rock and the underlying cavity is unstable, and a plurality of rock bolts are installed at the interface between the remaining mass of rock and the cavity in order to help stabilize the interface. The rock bolts stabilize the interface by transferring load from the unstable exterior of the rock mass at its interface with the cavity to the interior of the rock mass, which is much stronger and more stable.
An individual rock bolt can be one to twelve meters in length and is installed in an exposed rock face from the cavity side by boring a hole in the rock mass that is at least as long as the rock bolt and of a slightly bigger diameter. The rock bolt is placed in the hole, and a mechanism within the rock bolt is engaged to anchor the rock bolt into the hole towards the distal end of the rock bolt. The anchored rock bolt is then sealed within the hole by applying cementing grout into the hole, which hardens to seal the rock bolt in place. Not only does the cementing grout seal the rock bolt in place, the cementing grout acts as a coating for the metal parts of the rock bolt within the bore hole which protects these parts of the rock bolt from corrosion. A typical rock bolt which may be installed in this manner is shown in U.S. Pat. No. 5,636,945. Such a rock bolt is commercially available under the trademark CT BOLT® from Vik Oersta A/S—Rock Support, Strandgata 59, N-6151 Oersta, Norway.
Before hardening, the cementing grout is a runny paste which is injected into the rock bolt in such a manner (discussed in greater detail below in the Detailed Description of the Invention) that the bored hole surrounding the rock bolt is filled with the cementing grout. The rock bolt comprises a flange or washer plate towards its proximal end, with the flange or washer plate resting adjacent to the rock face, and the purposes of the flange or washer plate are (1) to position the rock bolt in place with its distal end in the hole and its proximal end outside the hole prior to engagement of the anchor mechanism and insertion of the cementing grout and (2) to help prevent the cementing grout from running out of the hole before the cementing grout hardens.
Unfortunately, the flange or washer plate does not do an adequate job at preventing the cementing grout from running out of the hole. As previously explained, the rock face is the interface between the rock remaining after rock is removed during excavation and the cavity created by the excavation. It is naturally a roughened surface, unless ground down or cemented over with a material such as shotcrete. Gaps will exist between the flange or washer plate and the rock face (or the rock face with the surface coating of the material such as shotcrete), and cementing grout can escape through these gaps before the cementing grout hardens. For ease of reference, the term “rock face” will be used herein to refer to a face of exposed rock and a face of exposed rock covered with a coating of a material such as shotcrete interchangeably.
Since the cementing grout is inserted into the hole through the rock bolt under pressure, cementing grout escaping through the gaps creates a mess at the worksite during installation of rock bolts. The mess created by the escaped cementing grout goes beyond the aesthetic. Bore holes into which cementing grout is inserted are often located in a rock mass overhead of where workers are installing the rock bolts and applying the cementing grout. Escaped cementing grout will land on the workers and worksite underneath, which is a health and safety hazard. Work must be stopped after a cementing grout leak to clean up the workers and worksite in order to eliminate the hazard of the escaped cementing grout. It is always a goal in any business to prevent, or at least lower the possibility of unnecessary work stoppages and workplace hazards, so practitioners in the field have tried to find a solution to the problem of cementing grout escaping during the grouting process during installation of rock bolts.
Practitioners in the field have previously tried to plug these gaps with rags at the first sign of cementing grout escaping through a gap. This solution is unsatisfactory since it is reactionary—one must detect escaping cementing grout before a gap is plugged with a rag. Time is taken away from the job of installing a rock bolt when one has to detect and plug leaks manually, and further time is taken away since one still has to clean up the cementing grout that has leaked out of the gaps prior to insertion of the rag plug. Another solution is to apply a resin-based substance to fill gaps between the flange or washer plate and the rock face prior to insertion of the cementing grout. Such a substance that has been used in the past is made of an isocyanate polyol resin with flame retardants and amine catalysts added. This substance is inserted in the gaps as a liquid which forms an expanding resin-based foam which fills the gaps. While this solution is proactive since it fills the gaps before cementing grout is installed and can escape through gaps, this solution is also unsatisfactory since one must wait for the substance to harden before cementing grout can be installed, thereby increasing the time taken to apply cementing grout by making the grouting process a two-step process. Workers first have to place a rock bolt in a bore hole and fill gaps with the resin-based substance, wait for the substance to harden and then, in a second step, revisit the rock bolt after the substance has hardened to insert the cementing grout. Such a two-step process with installers revisiting the workspace a second time increases expenses for an installing contractor and always introduces the possibility that certain rock bolts are missed in the second step and are not cemented in place with cementing grout.
The present invention provides a seal for placement between the flange or washer plate and the rock face. The seal will plug potential leaks proactively in an installation process wherein the installing workers do not have to wait for the seal to harden before inserting the cementing grout.